Methods of obtaining perpetual clear images through electrostatic charge involve developing an electrostatic charged image formed on an electrophotosensitive material or electrostatic recording medium, using toner which has previously been electrostatically charged by friction, and then fixing the image. In the case of magnetic latent images, a latent image on a magnetic drum is developed by toner containing a magnetic substance, and then fixed.
The fixing is accomplished either by direct fusion of a developed toner image onto an electrophotosensitive material or electrostatic recording medium, or by transferring a toner image onto paper or film and then fusing it onto a transfer sheet. The toner fusion is accomplished by contact with a solvent vapor, or by pressure and heat. The heating system may be either a non-contact heating system employing an electrical oven or a contact heating system employing a contact roller, but the latter has been mainly used in recent years due to demand for a faster fixing process.
Toner used in dry developing processes include one-component toner and two-component toner. To prepare a two-component toner, first a resin, coloring agent, charge controller and other necessary additives are melted, kneaded and made into a thorough dispersion which is coarsely and then finely crushed, and sorted into prescribed grain size ranges. A one-component toner is prepared in the same manner, though with addition of magnetic iron powder to each of the components of the above-mentioned two-component toner.
The resin, being the major component of the toner mixture, is responsible for most of the required performance of the toner. Thus, for toner production, toner resins must have good dispersability in the coloring agent during the melting/kneading process and good crushability during the crushing process, and for toner use they require a variety of properties, including satisfactory fixation properties, non-offsetting properties, blocking resistance and electrical qualities.
Publicly known resins used for toner production include epoxy resins, polyester resins, polystyrene resins, methacrylic resins and the like, but contact heat fixation systems primarily use styrene and (meth)acrylic acid ester copolymers. Recently, however, attention has been focused on polyester resins because they allow fixation at even lower temperatures and provide excellent polyvinyl plasticizer resistance for the fixed toner images.
Also, in order to obtain color images, the colors must be produced and fixed by adhering 3- or 4-color toner onto transfer paper during the developing process described above, and then melting and mixing each of the components in the fixing process. It is strongly preferred, as mentioned above, that binder resins for full color toner have good mixability during the fixing process, i.e., good melt flowability. However, using binder resins with satisfactory melt flowability results in the problem of an offsetting phenomenon during the fixing process.
Although this offsetting phenomenon may be prevented by crosslinking the binder resin to induce polymerization, such a measure lowers its melt flowability and thus renders it unsuitable as a binder resin for full color toner. Consequently, in order to prevent offsetting when using full color copiers, silicone oil or the like is applied onto the surface of the fixing roller.
In addition, recent emphasis has been placed not only on image properties but also on environmental factors, and the generation of irritating odors during toner production and during copying has led to a number of problems with operators and at offices. Furthermore, since full color toners are often used for copying to OHP sheets, the problem results that the permeability of the copy material is lowered when the binder resin is colored, and the image is hazy when projecting by OHP. Moreover, coloring of binder resins when using full color toners employing a mixture of the 3 primary colors cyan, magenta and yellow, results in a narrow range of color tone reproduction, and thus lower image precision of the copied print. Thus, under the present circumstances in which even higher image precision is in demand, binder resins are now required which have high performance, i.e., low odor and low coloring.